I run the NGK BKR6E in my 505 , same plug in my 440 . 505 is 11:1. Never any issues . Under 10:1 NA I would run -5 These are projected plugs. Only thing with NGK's is they hate being over fuelled .
Tex
New best ET 10.259@129.65 . New best MPH 130.32 Finally fitted a solid cam, stepped it up a bit more 3690lbs through the mufflers New World block 3780lbs 10.278@130.80 . Wowser 10.253@130.24 footbraking from 1500rpm Power by Tex's Automotive